Do the Eyes Have It?

We all know the adage that dogs are man’s best friend. And we’ve all heard heartwarming stories about dogs who save their owners—waking them during a fire or summoning help after an accident. Anyone who has ever loved a dog knows the amazing, almost inexpressible warmth of a dog’s companionship and devotion. But it just might be that dogs have done much, much more than that for humankind. They may have saved not only individuals but also our whole species, by “domesticating” us while we domesticated them.

One of the classic conundrums in paleoanthropology is why Neandertals went extinct while modern humans survived in the same habitat at the same time. (The phrase “modern humans,” in this context, refers to humans who were anatomically—if not behaviorally—indistinguishable from ourselves.) The two species overlapped in Europe and the Middle East between 45,000 and 35,000 years ago; at the end of that period, Neandertals were in steep decline and modern humans were thriving. What happened?

A stunning study that illuminates this decisive period was recently published in Science by Paul Mellars and Jennifer French of Cambridge University. They argue, based on a meta-analysis of 164 archaeological sites that date to the period when modern humans and Neandertals overlapped in the Dordogne region of southwest France, that the modern-human population grew so rapidly that it overwhelmed Neandertals with its sheer numbers.

Because not all the archaeological sites in the study contained clearly identifiable remains of modern humans or Neandertals, Mellars and French made a common assumption: that sites containing stone tools of the Mousterian tradition had been created by Neandertals, and those containing more sophisticated and generally later stone tools of the Upper Paleolithic were made by modern humans. This link between tool and toolmaker is well supported by sites that do contain hominin remains, but there is nothing inherent in a stone tool that tells you who made it—not even if you find a skeleton right next to it. Still, stone tools are one of the best available indicators of which species—modern human or Neandertal—inhabited a particular location.

Mellars and French compared the number and sizes of Neandertal and modern-human archaeological sites, as well as the density of tools and the weight per square meter of prey animals, represented by fossils, in those sites. They standardized their results for 1,000-year periods to compensate for the varying amounts of time that the different locations had been occupied. In every respect, modern humans surpassed Neandertals. In fact, the greater success of modern humans was so clear that, according to Mellars and French’s calculations, the human population increased tenfold over the 10,000-year overlap period. Modern humans thrived and Neandertals did not—even though Neandertals had lived in the European habitat for about 250,000 years before modern humans “invaded.” Why weren’t Neandertals better adapted to their environment than the newcomers?

There is no shortage of hypotheses. Some favor climate change, others a modern-human advantage derived from the use of more advanced hunting weapons or greater social cohesion. Now, several important and disparate studies are coming together to suggest another answer, or at least another good hypothesis: The dominance of modern humans could have been in part a consequence of domesticating dogs—possibly combined with a small, but key, change in human anatomy that made people better able to communicate with dogs.